Liberal whose activity is set to Teaching Other Liberals (Teacher) is able to do so if

There are other Liberals at Teachers location

… who have less skill than Teachers skill - 1

… and who have less skill than Teachers Teaching skill + 2

Also, the amount of students has a negative impact on teaching efficiency, when there is more than 10 valid students per skill. However, a Teacher with skill X at 10 and Teaching at 10 (presumably quite common situation) is yet to be hampered by the impact when skill-less student amount hits 30.